SERS enhancement

The use of near-IR-laser excited FT-SERS eliminates the disturbing fluorescence of impurities found with visible excitation, and provides SERS enhancement factors that are about 20 times larger than those found for excitation at 514.5nm [792]. For a strong Raman scatterer (fluorene), a typical detection limit of 500 ng is found for a 3-mm diameter spot. For weak scatterers, the detection limits may be in the high- xg region, which means that some compromise between chromatographic... 

Figure 10.2. Electron micrographs of typical colloidal gold and silver particle structures used in SERS experiments. (a) Colloidal gold particles in the isolated and aggregated stage after addition of NaCl. (b) Typical colloidal silver clusters exhibiting strong SERS enhancement. (With permission from Refs. 17 and 18.)...

Kneipp et al. [34] showed that enhancement is independent of cluster sizes ranging from 100 nm to 20 pm. The data and the electron microscope images of the SERS particles are depicted in Figure 10.8 together with the nonresonance SERS spectrum of 10 6 M crystal violet. SERS enhancement is estimated to be on the order of 106 for the spatially isolated cluster and up to 108 for the colloidal clusters. The isolated silver clusters were made by the laser ablation technique mentioned earlier. 

The molecule is often represented as a polarizable point dipole. A few attempts have been performed with finite size models, such as dielectric spheres [64], To the best of our knowledge, the first model that joined a quantum mechanical description of the molecule with a continuum description of the metal was that by Hilton and Oxtoby [72], They considered an hydrogen atom in front of a perfect conductor plate, and they calculated the static polarizability aeff to demonstrate that the effect of the image potential on aeff could not justify SERS enhancement. In recent years, PCM has been extended to systems composed of a molecule, a metal specimen and possibly a solvent or a matrix embedding the metal-molecule system in a molecularly shaped cavity [62,73-78], In particular, the molecule was treated at the Hartree-Fock, DFT or ZINDO level, while for the metal different models have been explored for SERS and luminescence calculations, metal aggregates composed of several spherical particles, characterized by the experimental frequency-dependent dielectric constant. For luminescence, the effects of the surface roughness and the nonlocal response of the metal (at the Lindhard level) for planar metal surfaces have been also explored. The calculation of static and dynamic electrostatic interactions between the molecule, the complex shaped metal body and the solvent or matrix was done by using a BEM coupled, in some versions of the model, with an IEF approach. 

As a sensitive analytical tool, SERS spectroscopy can greatly increase both sensitivity and specificity of Raman spectroscopy. Hitherto, it has been considered that two mechanisms should be responsible for the SERS enhancement. 

Finally, the SERS Enhancement Factor is predicted [1] to be of the order of larger than say larger by a factor of — 10 —10. But... 

The RIE samples produced using this technique were formed over areas of cm. These samples could easily be broken up to create dozens of samples ready for batch-coating with metal, thus providing an efficient means of substrate production. Their SERS enhancement factors were determined in the same manner as for the butterfly wing substrates, and were found to be of similar magnitude. 

Sections of wing samples from Cicadetta cells cicadas were coated in a layer of high purity (99.99%) silver 60 nm thick using a Kurt J Lesker CMS-18HV electron beam deposition system controlled by a quartz crystal microbalance. These cicada wing substrates were found to produce a SERS enhancement factor of 10 when self-assembled monolayers of thiophenol were formed on their metalized surfaces. More recent work by Kostovski et al. has focused on... 

There are two types of SERS mechanisms, which are responsible for the observation of the SERS enhancement [8] one type is the long-range EM effect and the other is the short-range CHEM effect. The EM effect is believed to be the result of localized surface plasmon resonance electric fields (hot spot) set up onto the roughened metallic surfaces [9, 10, 31]. The probe molecules residing within these hot spots will be strongly excited and subsequently emit amplified Raman... 

Chowdhury et al. demonstrated Rhodamine 123 (Rh 123) monolayer organized on the Ag island film by the LB techniques. Results showed that the Rh 123 molecules in a monolayer on the Ag island film offered a high SERS activity. The huge SERS enhancement intensity confirms the charge transfer interaction between the Rh 123 and the Ag island film by physisorption of Rh 123 on the Ag island film [48]. 

Recently, Sun et al. [60] proposed to fabricate Ag nanostructures by electrochemical deposition of Ag in anodic aluminum oxide templates with each pore diameter of 100 nm. The morphology of Ag substrates was characterized by EESEM. The length of the Ag nanowires is estimated to be about 2 pm from the EESEM images. In addition, the SERS enhancement factor calculated for Ag nano wires SERS substrates is approximately 10 in magnitude. The Ag nano wire arrays are expected to have important applications in the development of high sensitivity SERS-based substrates. 

Luo et al. [61] demonstrated a simple method for the preparation of SERS active Ag nanostructures substrates by deposition of Ag nanoparticles into the designed Si holes. The morphologies of the Ag nanostructures were observed with SEM. The diameters of the Ag nanoparticles were found to be 40-60 nm. With increasing deposition time, flower-like Ag nanostructure commenced crystallization to form near the edge of the bottom surface of the Si microholes. These Ag nanostructures exhibited strong SERS enhancement, which provided an excellent platform for monitoring the R6G molecules by SERS technology [62]. 

Zoval JV, Biemacki P, Penner RM (1996) Implementation of electrochemically synthesized silver nanocrystallites for the preferential SERS enhancement of defect modes on thermally etched Graphite surfaces. Anal Chem 68 1585... 

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